Numerous kinds of vehicles having retractable jacks for stabilization and/or lifting are known in the art and are used in a wide range of applications. Typically, the stabilizing jacks are hydraulically operated and are moveable between retracted and extended positions. When in the retracted position, the stabilizing jacks are out of the way and allow the vehicle to move about without interference from the jacks. When in the extended position, the stabilizing jacks contact the ground and support at least a portion, if not the entirety, of the vehicle. In certain applications, the jacks may be used merely to stabilize the vehicle, whereas in other applications, the jacks my lift all or a portion of the vehicle to level the vehicle or otherwise position the vehicle in a desired attitude.
While such stabilizing jack systems may be manually controlled, many jack systems are partially- or fully-automated, and use a jack deployment system to automatically extend or deploy the jacks until they provide the desired degree of lift or stabilization. In such an application, the jack deployment system may utilize one or more tilt sensors to detect or sense the tilt angle or attitude of the vehicle. The jack deployment system extends the various vehicle jacks until the tilt sensors indicate that the vehicle has been leveled or has otherwise reached the desired attitude.
Unfortunately, however, such jack deployment systems are not without their drawbacks. For example, one problem that can arise relates to the sensitivity of the tilt sensors used to sense the attitude or tilt angle of the vehicle. If the sensors are too sensitive compared with the ability of the jack deployment system to position the vehicle within a certain tolerance, the system may have difficulty achieving the desired tilt or attitude set point. The system may “hunt” excessively in an attempt to achieve the attitude set point. While this problem can be overcome by re-calibrating the tilt sensors (i.e., to change the scaling factors); such re-calibration reduces the resolution of the sensors, thereby reducing the leveling accuracy of the system. Indeed, in systems involving such re-calibrated sensors, it is not unusual for the actual leveling accuracy to be many times lower than the design accuracy. For example, a system designed to level within 0.1 degree may actually only level to within 0.4 degree.
Another solution is to filter the output of the tilt sensor to smooth-out the signal. Unfortunately, however, such filtering may cause the jack deployment system to over- or under-shoot the desired attitude set point. If this occurs, the jack deployment system may repeatedly tilt the vehicle back and forth in an attempt to achieve the desired attitude set point. Besides increasing the amount of time required for the system to achieve the desired attitude, such back and forth vehicle motion can impose excessive stress on the vehicle. Even worse, if the system is not properly damped, there is a danger that the back and forth motion will cause the vehicle to enter a harmonic vibration state. Such harmonic vibrations are of particular concern if the vehicle is provided large, mast-like structures, such as a drill derrick.
In addition to the structural problems caused by such repeated back and forth vehicle motion, such motion can result in excessive settling of the vehicle on the supporting ground, which can further increase the difficulty in achieving the desired attitude set point. Indeed, in some cases the overall system hysteresis caused by jack settling can make it impossible for the jack deployment system to ever achieve the desired attitude set point.
Still other problems may stem from the particular speed at which the jacks are extended. For example, a fast jack extension speed increases the likelihood that the jack deployment system will overshoot the desired set point. As described above, such overshooting of the desire set point may lead to excessive back and forth vehicle motion, increased vehicle stress, the danger of inducing harmonic vibrations, and settling hysteresis, all of which are deleterious. Moreover, fast jack extension speeds may require rapid cycling of the jack actuators as the vehicle nears the desired attitude set point. Such rapid cycling can impose excessive stresses on the jacks and may also induce harmonic vibrations in the vehicle structure.
On the other hand, if the jack extension speed is too slow, the system may require an excessive period of time to achieve the desired attitude set point, particularly if the initial vehicle attitude departs significantly from the desired attitude set point. In addition, undershooting the desired set point may also result in undesirable back and forth vehicle motion as the jack extension system attempts to reach the desired attitude set point.